Probing magnetic anisotropy and exchange bias in coupled Au-Fe$_{3}$O$_{4}$ nanoclusters

The study of magnetic anisotropy and exchange bias in coupled nanoparticle systems is of topical interest. We have demonstrated radio frequency (RF) transverse susceptibility (TS) using a sensitive, self-resonant tunnel-diode oscillator (TDO) technique developed by us to be excellent for probing magnetic anisotropy and exchange bias (EB) in Fe$_{3}$O$_{4}$ particles grown epitaxially on one or multiple facets of polyhedral Au seed particles forming dumbbell- or flower-shaped Au-Fe$_{3}$O$_{4}$ nanoclusters. TS experiments reveal a strong increase in magnetic anisotropy in coupled Au-Fe$_{3}$O$_{4}$ nanoclusters compared to pure Fe$_{3}$O$_{4}$ nanoparticles. TS experiments also probe a surface spin glass transition (T$_{F})$, a sharp increase in surface anisotropy at T$_{F}$, and a strong increase in EB with temperature below T$_{F}$ in the flower-shaped nanoclusters. Our RF susceptibility measurements are in good agreement with conventional AC and DC magnetometry. The influence of the Au interface(s) on the surface spin configuration of Fe$_{3}$O$_{4}$ nanoparticles will be discussed.